统计不同回文子序列

/*
 * Approach:
 * Using Java Streams, we can calculate the number of distinct palindromic subsequences by first identifying
 * all subsequences of the string and then filtering out the palindromic ones. We use a set to keep track of
 * distinct palindromic subsequences and return the size of the set.
 */
import java.util.HashSet;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class SolutionStream {
    private static final int MOD = 1_000_000_007;

    public int countPalindromicSubsequences(String s) {
        Set<String> palindromicSubsequences = IntStream.range(0, s.length())
            .boxed()
            .flatMap(i -> IntStream.range(i + 1, s.length() + 1)
                .mapToObj(j -> s.substring(i, j)))
            .filter(this::isPalindrome)
            .collect(Collectors.toSet());

        return palindromicSubsequences.size() % MOD;
    }

    private boolean isPalindrome(String str) {
        int left = 0, right = str.length() - 1;
        while (left < right) {
            if (str.charAt(left++) != str.charAt(right--)) {
                return false;
            }
        }
        return true;
    }
}